The present invention relates generally to the field of separators for electrochemical cells. More particularly, this invention pertains to separators for electrochemical cells which comprise at least one microporous pseudo-boehmite layer and at least one protective coating layer comprising a polymer; electrolyte elements comprising such separators; electric current producing cells comprising such separators; and methods of making such separators, electrolyte elements, and cells.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more filly describe the state of the art to which this invention pertains.
In an electric current producing cell or battery, discharge of the cell from its charged state occurs by allowing electrons to flow from the anode to the cathode through an external circuit resulting in the electrochemical reduction of the cathode active material at the cathode and the electrochemical oxidation of the anode active material at the anode. Under undesirable conditions, electrons may flow internally from the anode to the cathode, as would occur in a short circuit. To prevent this undesirable internal flow of electrons that occurs in a short circuit, an electrolyte element is interposed between the cathode and the anode. This electrolyte element must be electronically non-conductive to prevent the short circuiting, but must permit the transport of positive ions between the anode and the cathode during cell discharge, and in the case of a rechargeable cell, during recharge. The electrolyte element should also be stable electrochemically and chemically towards both the anode and the cathode.
Typically, the electrolyte element contains a porous material, referred to as a separator since it separates or insulates the anode and the cathode from each other, and an aqueous or non-aqueous electrolyte in the pores of the separator. The aqueous or non-aqueous electrolyte typically comprises ionic electrolyte salts and electrolyte solvents, and optionally, other materials such as for example, polymers. A variety of materials have been used for the porous layer or separator of the electrolyte element in electric current producing cells. These porous separator materials include polyolefins such as polyethylenes and polypropylenes, glass fiber filter papers, and ceramic materials. Usually these separator materials are supplied as porous free standing films which are interleaved with the anodes and the cathodes in the fabrication of electric current producing cells. Alternatively, the porous separator layer can be applied directly to one of the electrodes, for example, as described in U.S. Pat. No. 5,194,341 to Bagley et al.
Porous separator materials have been fabricated by a variety of processes including, for example, stretching combined with special heating and cooling of plastic films, extraction of a soluble plasticizer or filler from plastic films, and plasma oxidation. The methods for making conventional free standing separators typically involve extrusion of melted polymeric materials either followed by a post-heating and stretching or drawing process or followed by a solvent extraction process to provide the porosity throughout the separator layer. U.S. Pat. No. 5,326,391 to Anderson et al. and references therein, describe the fabrication of free standing porous materials based on extraction of a soluble plasticizer from pigmented plastic films. U.S. Pat. No. 5,418,091 to Gozdz et al. and references therein, describe forming electrolyte layers by extracting a soluble plasticizer from a fluorinated polymer matrix either as a coated component of a multilayer battery structure or as an individual separator film. U.S. Pat. No. 5,194,341 to Bagley et al. describes an electrolyte element with a microporous silica layer and an organic electrolyte. The silica layer was the product of the plasma oxidation of a siloxane polymer. These manufacturing methods for free standing separators are complex and expensive and are not effective either in providing ultrafine pores of less than 1 micron in diameter or in providing separator thicknesses of less than 15 microns.
The methods for making a separator coated directly on another layer of the cell typically involve a solvent extraction process after coating to provide the porosity throughout the separator layer. As with the free standing separators, this solvent extraction process is complex, expensive, and not effective in providing ultrafine pores of less than 1 micron in diameter.
Carlson et al. in U.S. patent application Ser. No. 08/995,089 to the common assignee, describe separators for use in electrochemical cells which comprise a microporous pseudo-boehmite layer and electrolyte elements comprising such separators. The pseudo-boehmite separators and methods of making such separators are described for both free standing separators and as a separator layer coated on an electrode.
As the non-aqueous electrolyte in the pores of the separator of an electrolyte element, a liquid organic electrolyte comprising organic solvents and ionic salts is typically used. Alternatively, a gel or solid polymer electrolyte containing polymers and ionic salts, and optionally organic solvents, might be utilized instead of the liquid organic electrolyte. For example, U.S. Pat. Nos. 5,597,659 and 5,691,005 to Morigaki et al. describe a separator matrix formed of a microporous polyolefin membrane inpregnated in its pores with an ionic conductive gel electrolyte.
In addition to being porous and being chemically stable to the other materials of the electric current producing cell, the separator should be flexible, thin, economical in cost, and have good mechanical strength. These properties are particularly important when the cell is spirally wound or is folded to increase the surface area of the electrodes and thereby improve the capacity and high rate capability of the cell. Typically, free standing separators have been 25 microns (xcexcm) or greater in thickness. As batteries have continued to evolve to higher volumetric capacities and smaller lightweight structures, there is a need for separators that are 15 microns or less in thickness with a substantial increase in the area of the separator in each battery. Reducing the thickness from 25 microns to 15 microns or less greatly increases the challenge of providing porosity and good mechanical strength while not sacrificing the protection against short circuits or not significantly increasing the total cost of the separator in each battery.
This protection against short circuits is particularly critical in the case of secondary or rechargeable batteries with lithium as the anode active material. During the charging process of the battery, dendrites can form on the surface of the lithium anode and can grow with continued charging. A key feature of the separator in the electrolyte element of lithium rechargeable batteries is that it have a small pore structure, such as 10 microns or less in pore diameter, and sufficient mechanical strength to prevent the lithium dendrites from contacting the cathode and causing a short circuit with perhaps a large increase in the temperature of the battery leading to an unsafe explosive condition.
Another highly desirable feature of the separator in the electrolyte element is that it is readily wetted by the electrolyte which provides the ionic conductivity. When the separator material is a polyolefinic material, which has nonpolar surface properties, the electrolytes (which typically have highly polar properties) often poorly wet the separator material. This results in low capacities in the battery due to the nonuniform distribution of the electrolyte in the electrolyte element.
Further it would be highly advantageous to be able to prepare free standing separators by a relatively simple process of coating which directly provides ultrafine pores as small as 1 nm in diameter and can readily provide a range of thicknesses from 50 microns or greater down to 1 micron. Also, it would be advantageous to be able to prepare separators with ultrafine pores and a wide range of thicknesses coated directly on another layer of the electric current producing cell by a process of coating without requiring any subsequent solvent extraction or other complex process which is costly, difficult to control, and not effective in providing ultrafine pores.
A separator, particularly one with a thickness less than 25 microns, which is applicable for electric current producing cells, and which can avoid the foregoing problems often encountered with the use of polyolefinic and other conventional porous materials made using extrusion, extraction, or other processes would be of great value to the battery industry.
The present invention pertains to a separator for use in an electric current producing cell, wherein the separator comprises (i) at least one microporous pseudo-boehmite layer in contact with (ii) at least one protective coating layer comprising a polymer. In one embodiment, the protective coating layer is adjacent to one outer surface of the microporous pseudo-boehmite layer. In one embodiment, the protective coating layer is an intermediate layer between two microporous pseudo-boehmite layers, wherein the compositions of the microporous pseudo-boehmite layers may be the same or different. In one embodiment, the protective coating layer is an intermediate layer between two microporous pseudo-boehmite layers, and the separator further comprises an additional protective coating layer on the outside surface of one or both microporous pseudo-boehmite layers, and further wherein the compositions of the two microporous pseudo-boehmite layers may be the same or different, and the compositions of the two or more protective coating layers may be the same or different. In one embodiment, the microporous pseudo-boehmite layer is an intermediate layer between two protective coating layers, wherein the compositions of the protective coating layers may be the same or different.
In one embodiment of the invention, the polymer of the protective coating layer comprises one or more moieties from the polymerization of one or more monomers or macromonomers selected from the group consisting of: acrylates, methacrylates, olefins, epoxides, vinyl alcohol, vinyl ethers, and urethanes. In one embodiment, the olefinic monomer is selected from the group consisting of: ethylene, propylene, butene, pentene, hexene, octene, and styrene. In one embodiment, the acrylate monomer or macromonomer is selected from the group consisting of: polyethylene glycol diacrylates, polypropylene glycol diacrylates, ethoxylated neopentyl glycol diacrylates, ethoxylated bisphenol A diacrylates, ethoxylated aliphatic urethane acrylates, ethoxylated alkylphenol acrylates, and alkylacrylates.
In another embodiment, the polymer of said protective coating layer comprises one or more moieties formed by polymerization of one or more monomers or macromonomers selected from the group consisting of monomers and macromonomers having the formula:
R1(R2O)nxe2x80x94R3
wherein:
R1 is the same or different at each occurrence and is selected from the group consisting of:
CH2xe2x95x90CH(Cxe2x95x90O)xe2x80x94Oxe2x80x94,
CH2xe2x95x90C(CH3)(Cxe2x95x90O)Oxe2x80x94,
CH2xe2x95x90CHxe2x80x94, 
CH2xe2x95x90CHxe2x80x94Oxe2x80x94;
R2 is the same or different at each occurrence and is selected from the group consisting of:
xe2x80x94CH2xe2x80x94CH2xe2x80x94,
xe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94,
xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94,
xe2x80x94CH(C2H5)xe2x80x94CH2xe2x80x94,
xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94,;
R3 is the same or different at each occurrence and is selected from the group consisting of:
methyl, ethyl, propyl, isopropyl, butyl, isobutyl, hexyl, 2-ethylhexyl, decyl, dodecyl, phenyl, butlyphenyl, octylphenyl, nonylphenyl, R1, xe2x80x94Xxe2x80x94(OR2)mxe2x80x94R1, xe2x80x94Y[(OR2)oxe2x80x94R1]2, xe2x80x94Z[(OR2)pxe2x80x94R1]3;
X is a divalent radical selected from the group consisting of: 
xe2x80x94(CH2)rxe2x80x94, where r is 3, 4, or 6;
Y is a trivalent radical selected from the group consisting of: 
Z is a tetravalent radical selected from the group consisting of: 
m is an integer ranging from 0 to 100;
n is an integer ranging from 0 to 100;
o is an integer ranging from 0 to 100; and,
p is an integer ranging from 0 to 100.
In a preferred embodiment, the polymer has a molecular weight of greater than 10,000. In a more preferred embodiment, the polymer has a molecular weight greater than 50,000.
In one embodiment, the protective coating layer has a thickness of from about 0.2 microns to about 20 microns. In a preferred embodiment, the protective coating layer has a thickness of from about 0.5 microns to about 15 microns. In a more preferred embodiment, the protective coating layer has a thickness of from about 0.5 microns to about 10 microns. In a most preferred embodiment, the protective coating layer has a thickness of from about 0.5 microns to about 5 microns.
In one embodiment of the present invention, the protective coating layer further comprises a pigment. In one embodiment, the pigment of the protective coating layer is selected from the group consisting of: colloidal silicas, amorphous silicas, surface treated silicas, colloidal aluminas, amorphous aluminas, conductive carbons, graphites, tin oxides, titanium oxides and polyethylene beads.
In one embodiment, the polymer and the pigment are present in the protective coating layer at a weight ratio of from about 1:10 to about 10:1. In a preferred embodiment, the polymer and the pigment are present in the protective coating layer at a weight ratio of from about 1:4 to about 6:1. In a more preferred embodiment, the polymer and the pigment are present in the protective coating layer at a weight ratio of from about 1:3 to about 4:1.
In one embodiment, the pigment of the protective coating layer has a particle size of from about 1 nm to about 10,000 nm. In a preferred embodiment the pigment of the protective coating layer has a particle size of from about 2 nm to about 6,000 nm. In a more preferred embodiment, the pigment of the protective coating layer has a particle size of from about 5 nm to about 3,000 nm.
In another embodiment, the pigment of the protective coating layer has a particle size and the microporous pseudo-boehmite layer has an average pore diameter which is smaller than said particle size.
In one embodiment of the present invention, the pseudo-boehmite layer has a pore volume from 0.02 to 2.0 cm3/g. In a preferred embodiment, the pseudo-boehmite layer has a pore volume from 0.3 to 1.0 cm3/g. In a more preferred embodiment, the pseudo-boehmite layer has a pore volume from 0.4 to 0.7 cm3/g.
In one embodiment, the pseudo-boehmite layer of the separator has an average pore diameter from 1 to 300 nm. In a preferred embodiment, the pseudo-boehmite layer has an average pore diameter from 2 to 30 nm. In a more preferred embodiment, the pseudo-boehmite layer has an average pore diameter from 3 to 10 nm.
In one embodiment, the pseudo-boehmite layer of the separator has a thickness of from 1 micron to 50 microns. In a preferred embodiment, the pseudo-boehmite layer has a thickness of from 1 micron to 25 microns. In a more preferred embodiment, the pseudo-boehmite layer has a thickness of from 2 microns to 15 microns.
In another embodiment of the present invention, the pseudo-boehmite layer further comprises a binder. In one embodiment, the binder is present in an amount of 5 to 70% by weight of pseudo-boehmite in the pseudo-boehmite layer in the separator. In a preferred embodiment, the binder comprises polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, copolymers of the foregoing, or a combination thereof.
In one embodiment, the separator for use in an electric current producing cell comprises at least one microporous pseudo-boehmite layer in contact with at least one protective coating layer comprising a polymer and a silica. In a preferred embodiment, the silica of the protective coating layer is a hydrophobic silica.
Another aspect of the invention pertains to an electrolyte element for use in an electric current producing cell, the electrolyte element comprising: (a) a separator; and, (b) an organic electrolyte; wherein, the separator comprises: (i) at least one microporous pseudo-boehmite layer, as described herein, in contact with (ii) at least one protective coating layer comprising a polymer, as described herein; and the organic electrolyte is contained within pores of the separator. Suitable materials for use as the organic electrolyte include liquid electrolytes, gel polymer electrolytes, and solid polymer electrolytes. In a preferred embodiment, the organic electrolyte is a liquid electrolyte.
Still another aspect of the present invention pertains to a method of forming a separator for use in electric current producing cells, wherein the separator comprises: (i) at least one microporous pseudo-boehmite layer, as described herein, in contact with (ii) at least one protective coating layer comprising a polymer, as described herein, the method comprising the steps of: (a) coating onto a substrate a first liquid mixture, A, comprising a boehmite sol, or alternatively, coating onto a substrate a first liquid mixture, B, comprising one or more polymers, monomers, or macromonomers, to form a first coating layer; (b) drying the first coating layer formed in step (a) to form a microporous pseudo-boehmite layer, if the first liquid mix A was utilized in step (a), or alternatively, drying the first coating layer formed in step (a) to form a protective coating layer, if the first liquid mixture B was utilized in step (a), to form a dried first coating layer, (c) coating onto the layer formed in step (b) a second liquid mixture, Bxe2x80x2; comprising one or more polymers, monomers, or macromonomers to form a second coating layer, if a microporous pseudo-boehmite layer was formed in step (b), or alternatively, coating onto the layer formed in step (b) a second liquid mixture, Axe2x80x2, comprising a boehmite sol, if a protective coating layer was formed in step (b), to form a second coating layer; (d) drying the second coating layer formed in step (c) to form a protective coating layer, if the second liquid mixture Bxe2x80x2 was utilized in step (c), or alternatively, to form a microporous pseudo-boehmite layer, if the second liquid mixture Axe2x80x2 was utilized in step (c), to form a dried second coating layer. In one embodiment, subsequent to formation of a protective coating layer, there is a further step of curing the dried coating layer to form a cured protective coating layer by use of an energy source. In one embodiment, the curing is performed using an energy source selected from the group consisting of: heat, ultraviolet light, visible light, infrared radiation, and electron beam radiation. In one embodiment, after step (d), steps (a) and (b) are repeated to form a third coating layer. In one embodiment, after step (d), steps (a), (b), (c), and (d) are repeated to form a third coating layer and a fourth coating layer.
In one embodiment, the polymers, monomers and macromonomers for use in forming the protective coating layer have a molecular weight which is too large for impregnation into pores of the microporous pseudo-boehmite layer. In one embodiment, the polymers, monomers and macromonomers have a molecular weight greater than 2000. In one embodiment, the polymers, monomers and macromonomers have a molecular weight greater than 5000.
In one embodiment of die method, the monomers and macromonomers of the first or second liquid mixture comprising polymers, monomers and macromonomers are selected from the group consisting of: acrylates, methacrylates, olefins, epoxides, vinyl alcohols, vinyl ethers, and urethanes. In one embodiment, the acrylate monomer or macromonomer of the first or second liquid mixture is selected from the group consisting of: polyethylene glycol diacrylates, polypropylene glycol diacrylates, ethoxylated neopentyl glycol diacrylates, ethoxylated bisphenol A diacrylates, ethoxylated aliphatic urethane acrylates, and ethoxylated alkylphenol acrylates.
In one embodiment, the monomers and macromonomers of the liquid mixtures, B or Bxe2x80x2, comprising one or more polymers, monomers or macromonomers are selected from monomers or macromonomers having the formula R1(R2O)nxe2x80x94R3, as described herein.
In one embodiment of the method, the liquid mixtures, B or Bxe2x80x2, comprising one or more polymers, monomers or macromonomers further comprises a second polymer. In one embodiment of the method, the liquid mixtures, B or Bxe2x80x2, comprising one or more polymers, monomers or macromonomers further comprises a pigment, as described herein.
In one embodiment, the liquid mixtures, B or Bxe2x80x2, comprising one or more polymers, monomers, or macromonomers has a viscosity from 15 cP to 5000 cP.
In one embodiment, the liquid mixtures, A or Bxe2x80x2, comprising a boehmite sol further comprises a binder, as described herein. In one embodiment, the binder is present in the amount of 5 to 70% of weight of the pseudo-boehmite in the pseudo-boehmite layer.
In one embodiment of the methods, subsequent to step (d), there is a further step of delaminating the separator from the substrate. In one embodiment, at least one outermost surface of the substrate comprises a cathode active layer and the first liquid mixture of step (a) is coated onto the cathode coating layer.
Yet another aspect of the present invention pertains to a method of making an electrolyte element for an electric current producing cell, wherein the electrolyte element comprises a separator comprising: (i) at least one microporous pseudo-boehmite layer in contact with (ii) at least one protective coating layer comprising a polymer; wherein the method comprises the steps of forming a separator, as described herein for methods of forming a separator, and after formation of a separator, there is a further step of contacting a surface of the separator with an organic electrolyte, as described herein, thereby causing infusion of the electrolyte into pores of the separator.
In a preferred embodiment of the method for making an electrolyte element, the organic electrolyte is a liquid electrolyte.
Still another aspect of the invention pertains to an electric current producing cell, said cell comprising a cathode, an anode, and an electrolyte element interposed between said cathode and said anode, wherein said electrolyte element comprises: (a) a separator, and, (b) an organic electrolyte; wherein, said separator comprises: (i) at least one microporous pseudo-boehmite layer, as described herein, in contact with (ii) at least one protective coating layer comprising a polymer, as described herein; and, said organic electrolyte, as described herein, is present within pores of said separator.
In one embodiment of the electric current producing cell, the cell is a secondary battery.
In one embodiment of the electric current producing cell, the anode active material is selected from the group consisting of: lithium metal, lithium-aluminum alloys, lithium-tin alloys, lithium-intercalated carbons, and lithium-intercalated graphites
In one embodiment of the electric current producing cell, the cathode comprises a cathode active material selected from the group consisting of electroactive transition metal chalcogenides, electroactive conductive polymers, and electroactive sulfur-containing materials.
In one embodiment of the electric current producing cell, the electroactive sulfur-containing material of the cathode comprises elemental sulfur. In one embodiment, the electroactive sulfur-containing material comprises a sulfur-containing polymer comprising a polysulfide moiety, Sm, selected from the group consisting of covalent xe2x80x94Smxe2x80x94 moieties, ionic xe2x80x94Smxe2x88x92 moieties, and ionic Sm2xe2x88x92 moieties, wherein m is an integer equal to or greater than 3. In one embodiment, m of the polysulfide moiety, Sm, of the sulfur-containing polymer is an integer equal to or greater than 8. In one embodiment, the sulfur-containing polymer has polymer backbone chain and the polysulfide moiety, Sm, is covalently bonded by one or both of its terminal sulfur atoms on a side group to the polymer backbone chain. In one embodiment, the sulfur-containing polymer has a polymer backbone chain and the polysulfide moiety, xe2x80x94Sm,xe2x80x94 is incorporated into the polymer backbone chain by covalent bonding of terminal sulfur atoms of the polysulfide moiety. In one embodiment, the sulfur-containing polymer comprises greater than 75 weight per cent of sulfur.
A further aspect of the present invention pertains to a method for forming an electric current producing cell. The method comprises providing an anode, as described herein, and a cathode, as described herein, and interposing an electrolyte element, as described herein, between the anode and the cathode. In one embodiment of the method for forming an electric current producing cell, the organic electrolyte of the electrolyte element comprises one or more materials selected from the group consisting of: liquid electrolytes, gel polymer electrolytes, and solid polymer electrolytes.
As will be appreciated by one of skill in the art, features of one aspect or embodiment of the invention are also applicable to other aspects or embodiments of the invention.